This invention relates to a process for preparing stable colloidal sols of antimony pentoxide in organic solvents (hereinafter referred to as "antimony pentoxide organosol"). More particularly, it relates to a novel process for preparing an antimony pentoxide organosol by processing an aqueous antimony pentoxide sol with an organic base and an organic acid and then replacing the dispersing medium of the sol from water to organic solvents.
The antimony pentoxide sol is used as an auxiliary of a flame retardant for plastics, fibers, etc.; as a microfiller for surface treatment for plastics, glasses, etc.; as an inorganic ion exchanger to remove a metallic ion; and the like.
At present, high concentration aqueous sol (Sb.sub.2 O.sub.5 content of 30 to 50% by weight and pH of 4 to 8) stabilized with an organic base has generally been used. However, the aqueous sol is used only with water or a hydrophilic organic solvent such as N,N-dimethylformamide (hereinafter referred to as "DMF") and alcohols, whereby its use is remarkably limited.
In recent years there has been an increased demand for methods of imparting flame retardancy to materials and especially polymers. This demand has been met using antimony pentoxide sols including sols in both hydrophilic organic solvent and hydrophobic organic solvents.
As the conventional processes for preparing the antimony pentoxide organosol, there are known (1) a method in which anhydrous antimony trioxide or antimony trichloride is added to nitric acid and after heating, .alpha.-hydroxycarboxylic acid is added thereto, then an organic solvent such as DMF is added thereto and water is removed by evaporation (U.S. Pat. No. 3,657,179); (2) a method in which to a monovalent or divalent or more of alcohols to be represented by ethylene glycol is added to a hydrogen halide such as hydrogen chloride, hydrophilic organic solvent such as DMF and .alpha.-hydroxycarboxylic acid to disperse antimony trioxide, and then oxidizing it with an aqueous hydrogen peroxide solution (U.S. Pat. Nos. 4,051,064 and 4,017,418); (3) a method in which antimony pentoxide sol, which was obtained by the method of oxidizing antimony trioxide with an aqueous hydrogen peroxide solution and stabilized with amine, is dried and pulverized, and then the powder is dispersed in an organic solvent such as DMF (U.S. Pat. No. 4,026,819); and (4) a method for preparing an aqueous sol stabilized with a combination of alkanol amine and .alpha.-hydroxycarboxylic acid, and phosphoric acid, etc. (U.S. Pat. No. 4,348,301) wherein there is disclosed that the aqueous sol obtained by this method is mutually soluble with a hydrophilic organic solvent (e.g., methanol, DMF, etc.).
The conventional methods as mentioned above are each limited to an organic solvent which has extremely high hydrophilic nature miscible with water at optional ratio. Also, since the antimony pentoxide organosols obtained by these methods contain extremely large amounts of .alpha.-hydroxycarboxylic acid (e.g., malic acid) or hydrohalogenic acid (hydrochloric acid), when they were added to the polymer for fibers to give flame resistance, problems such as lowering in physical properties of a polymer or corrosion of a recovery device due to particularly halogen on recovery of an organic solvent by evaporation after spinning are present.
An example of using a hydrophilic organic solvent organoantimony pentoxide sol is for flame resistant finishing of a polymer of acrylic fibers, or for microfillers in order to give a flame retardant property or increase a surface hardness to an alcohol solution of a silicone resin which has been used as a surface treatment for plastic films glasses, etc.
In U.S. Pat. No. 4,348,301 as mentioned above, it is possible to prepare a hydrophilic organic solvent sol in combination of alkanol amine and .alpha.-hydroxycarboxylic acid, and phosphoric acid, etc., but it cannot be used with a hydrophobic or non-aqueous organic solvent. Further, in U.S. Pat. No. 4,026,819, there is proposed a method in which an aqueous sol stabilized with an aliphatic amine, cyclic amine, etc. is prepared and dried, and the obtained powder is then dispersed in an organic solvent such as DMF to prepare an organosol. However, the organosol obtained by the method is unstable, and by this method, an organosol of a hydrophobic organic solvent such as toluene, etc. cannot be obtained.
In recent years, there are proposed methods in which antimony pentoxide is dispersed in a hydrophobic solvent by using a specific anionic surface active agent or nonionic surface active agent (Japanese Provisional Patent Publications Nos. 161729/1985, 161730/1985 and 161731/1985). However, these methods result in a great deal of aggregation of particles at a high concentration to thereby forming a slurry or gel. Therefore it is difficult to obtain an antimony pentoxide organosol using a hydrophobic solvent with low viscosity and high concentration. Further, the product having high concentration obtained by the method has disadvantage that change in color will sometimes occur with time.